Solid or phase change inks that are solid at ambient temperatures and liquid at elevated operating temperatures employed in ink jet printers have been utilized for an extended period of time. These printers eject liquid phase ink droplets from the print head at an elevated operating temperature. The droplets solidify quickly upon contact with the surface of the receiving substrate to form a predetermined pattern.
Among the advantages of solid ink is the fact that it remains in a solid phase at room temperature during shipping and long-term storage. Problems with clogging in the print head are largely eliminated, or are less prevalent than occur with aqueous based ink jet print heads. The rapid solidification or hardening of the ink drops upon striking the receiving substrates permits high quality images to be printed on a wide variety of printing media.
It is known that printed images formed from deformation of solid inks on receiving substrates during or following the printing process is possible. For example, U.S. Pat. No. 4,745,420 to Gerstenmaier discloses a solid ink that is ejected onto a receiving substrate and subsequently spread by the application of pressure to increase the coverage and minimize the volume of ink required. This has been used in direct solid ink printing. Deformation of solid ink drops also has occurred in direct printing as disclosed in U.S. Pat. No. 5,092,235 to Rise, where a high pressure nip defined by a pair of rollers applies pressure to cold fuse solid ink drops to receiving substrates.
An indirect printing process has been successfully employed with solid ink drops to apply droplets of solid ink in a liquid phase in a predetermined pattern by a print head to a liquid intermediate transfer that is supported by a solid support surface, and then transfer the solid ink after it hardens from the liquid intermediate transfer surface to a final receiving surface. Some deformation of the ink drops occur in the transfer process, as is described in U.S. Pat. No. 5,372,852 to Titterington et al.
Solid ink printing on transparencies has its resolution of the final printed image affected by the amount of light transmitted through the base media, any coatings on the media and the ink itself. Transparency materials can have an increased dynamic range, which is the difference between the maximum and minimum density, when compared with reflection hard copy materials such as paper. In order to achieve improved transmissivity, the lowest density materials must transmit as much light as possible. To accomplish this, the base media has as few components as possible so that the scattering of light passing through the media is minimized and the maximum amount of light can be transmitted rectilinearly through the medium. Use of solid or phase change ink in ink jet printers to make transparencies is known as evidenced by U.S. Pat. Nos. 4,801,473; 4,889,761; and 4,853,706.
In addition to creating transparencies with rectilinear light transmission, solid ink printer manufacturers have had to ensure that the ink has strong adherence to the base material. Various adhesions promoting coating have been applied to transparency basis to improve the adhesion of the solid ink to the media. These coatings are typically rough-textured on their exposed surface to create more bonding sites for the solid ink upon solidification. U.S. Pat. Nos. 4,992,304 and 5,110,665 address the use of adhesion promoting coatings on transparent substrates.
With the recent innovation of using solid ink to perform medical diagnostic imaging using multiple gray scale levels of black ink, there has been increased attention to creating a compatible adhesion promoting coating with the standard Mylar film used in x-ray medical diagnostic imaging employing silver halide. In addressing the problem of creating maximum transmittance of light to achieve the necessary contrast and imaging quality on the transparencies when they are viewed on a light box, it was anticipated from prior experience that the highest transmittance would be where there was an absence of printed ink, or what has been called "white space." The only materials through which light would pass in these non-imaged areas would be through the transparent media and the compatible adhesion promoting coating. Surprisingly, however, it was discovered that the rough surface of the coating itself caused light to scatter and thus not pass rectilinearly through the combined substrate adhesion promoting coating on the surface to thereby decrease the amount of light transmitted to an unacceptable level. The deflected light in the "white" areas was bent and scattered into adjacent imaged areas with the ink further reducing the quality of the image and image contrast.
These problems are solved in the present invention by the use of a clear or slightly tinted or colored wax base that is applied over the adhesion promoting coating adjacent to the imaged areas in what would have been the unoccupied or "white" space. The clear or slightly tinted wax base has a refractive index that is substantially the same as the refractive index of the adhesion promoting coating and thereby prevents the scattering of light rays that would have occurred as the light passed from the transparent substrate through the adhesion promoting coating. The light rays pass in a generally rectilinearly path through the media substrate, the adhesion promoting coating, and the clear or slightly gray wax base.